Cyclopalladation reactions have been extensively studied since 1965.3 Recently, Pd(II)-catalyzed C—H activation reactions have been shown to be compatible with a wide range of carbon-carbon and carbon-heteroatom bond-forming reactions through Pd(II)/Pd(0), Pd(II)/Pd(IV) and Pd(II)/Pd(II) catalytic cycles.4-6 However, versatile ligands that can promote both C—H cleavage and the subsequent functionalization steps remain scarce and are largely limited to mono-protected amino acids (MPAAs) and pyridine/quinoline based ligands.7 
As effective C—H functionalization often requires a synergistic relationship between ligand and substrate coordinated to the metal center, it is essential to develop ligands that match with a variety of directing groups so that the assembled complexes are reactive in cleaving C—H bonds of a number of substrate classes.7 This fact is exemplified in recent efforts towards developing broadly useful norbornene-mediated meta-selective C—H functionalizations, where previously developed ligand scaffolds have proven insufficient at promoting reactivity with two important classes of substrates: anilines and phenols. A general pathway for meta-arylation using norbornene as a mediator is shown schematically below, where “DG” is a directing group, Ln is a ligand, and Ar—I is an aryl iodide coupling partner.

The norbornene insertion step from the Catellani reaction8,9 has previously been successfully combined with palladium catalyzed C—H activation to achieve selective C—H functionalization of indoles.10,11 Recently, this key step was combined with ortho-directed C—H activation to achieve a net meta-functionalization via a relay process.12-14 Further development of this newly emerging meta-C—H functionalization strategy remains a significant, yet important challenge as it is complementary to other approaches for the direct meta-functionalization of arenes.15-23 